1. Field of the Invention
The invention relates to a fuel injection system, in particular a common rail system, for fuel delivery in internal combustion engines, in particular diesel engines of commercial vehicles, with a low pressure circuit in which at least one low-pressure feed pump supplies fuel from a fuel tank to a high-pressure pump which supplies the highly pressurized fuel to a central high-pressure fuel reservoir.
2. Description of the Prior Art
In diesel engines, air is taken in and compressed. At the end of the compression stroke, highly pressurized fuel is injected into the combustion chamber, where the mixture of fuel and air self-ignites.
In common rail fuel injection systems, the high-pressure pump, with the aid of the presupply pump, supplies the fuel to be injected from a tank to the central high-pressure fuel reservoir, which is referred to as the common rail. Fuel lines lead from the rail to the individual injectors, which are associated with the cylinders of the internal combustion engine. The motor electronics trigger the injectors individually as a function of the operating parameters of the internal combustion engine in order to inject fuel into the combustion chamber of the engine.
When the fuel tank is emptied or after maintenance work, the low-pressure circuit is filled with air. In conventional fuel injection systems, the air in the low-pressure circuit must be fed through the high-pressure pump into the high-pressure fuel reservoir, counter to the opening pressure (approx. 1.5 bar) of the intake valve of the high-pressure pump. From there, the air can escape via a return line. This venting counter to the opening pressure of the intake valve takes a very long time with conventional low-pressure feed pumps.
In order to accelerate the venting process, low-pressure feed pumps with a larger feed volume could be used. However, low-pressure feed pumps of this kind would be grossly oversized for normal operation. As a result, the fuel flow supplied by a low-pressure feed pump of this kind would have to be throttled during normal operation.
The object of the invention is to simplify the venting of the low-pressure circuit of a conventional fuel injection system. The fuel injection system according to the invention should also be simply designed and inexpensive to produce.
In a fuel injection system, in particular a common rail system, for supplying fuel to internal combustion engines, in particular diesel engines of commercial vehicles, with a low pressure circuit in which at least one low-pressure feed pump supplies fuel from a fuel tank to a high-pressure pump which supplies the highly pressurized fuel to a central high-pressure fuel reservoir, the object is attained by virtue of the fact that a device is provided for intentionally reducing the pressure in the low-pressure circuit.
As a result, the pressure in the low-pressure circuit can be reduced during venting. This offers the advantage that the air disposed in the low-pressure circuit can be displaced in a justifiable amount of time using a conventional low-pressure feed pump that is not oversized.
A particular embodiment of the invention is characterized in that the low-pressure circuit is connected to the tank by means of a return line in which a throttle is provided. The venting of air from the low-pressure circuit into the tank takes place by means of the throttle. This has the advantage that no large counterpressure has to be overcome during the venting.
Another particular embodiment of the invention is characterized in that the throttle is integrated into an overflow valve which is provided in the return line. The throttle can, for example, be embodied in the form of an intentionally leaky valve seat or as an additional throttle bore. Integrating the throttle into the overflow valve decreases costs.
Another particular embodiment of the invention is characterized in that a venting bore with a vent screw is disposed at the highest point of the low-pressure circuit after assembly, particularly in a metering unit for the high-pressure pump. The air in the low-pressure circuit can escape into the atmosphere via the manually operable vent screw. This embodiment type has the advantage that it is particularly inexpensive to produce because of its simplicity. The venting bore must be situated so that possibly escaping fuel can be collected in a container.
Another particular embodiment of the invention is characterized in that the venting screw has a flattening in the thread region, which permits fuel to pass through the venting bore when the venting screw is turned slightly. This prevents the fuel from spurting out in an uncontrolled manner at the beginning of the venting procedure.
Another particular embodiment of the invention is characterized in that the high-pressure pump is equipped with a metering device that has a solenoid valve which, when supplied with current, opens a connection between the low-pressure circuit and a return into the tank. Normally, the solenoid valve is closed while the internal combustion engine is being started. When the solenoid valve is supplied with current for a short time during venting, the air in the low-pressure circuit can escape into the tank.
Other advantages, characteristics, and details of the invention ensue from the following description in which three exemplary embodiments of the invention are described in detail in conjunction with the single drawing figure which shows a hydraulic connection diagram of a fuel injection system according to the invention.